1. Field of The Invention
This invention relates to an information reading device for use in, for example, a point-of-sales (POS) system in which slave devices such as a bar code reader and a keyboard are connected to a master unit such as a host system with a common signal line.
2. Description of The Related Art
As illustrated in FIG. 1, a conventional POS system has a configuration in which a large number of slave devices such as a bar code reader 1, a display 13 and a keyboard 12 are connected to a single master unit (namely, a host system 12) with common signal line (namely, a cable 11). Thus the slave devices communicate with the host system 12 through the cable 11. Further, communication interrupts from the host system 12 to each slave device are frequently caused by control commands output from the host system 12. For instance, in case where an operator sends a operation changing command from the keyboard 1 to the host system 12 in order to change an operation of a slave device, the host system 12 causes a communication interrupt by issuing a control command to the slave device in such a manner to change the operation of the slave device as intended by the operator.
Such a communication interrupt is caused by an interrupt signal output from the host system 12. Further, such communication interrupts to the slave devices, for example, the bar code reader 1, the display 13 and the keyboard 14 (incidentally, note that a practical POS system has a great number of slave devices) occur in the following cases. Namely, an interrupt from the host system 12 to the bar code reader 1 occurs when image data representing a bar code read by the reader 1 is sent therefrom to the host system 12. Further, an interrupt from the host system 12 to the keyboard 14 takes place when data input from such a data input device (namely, the keyboard 14) is sent to the host system 12.
When a communication interruption from the host system 12 to the bar code reader 1 of the POS system having such a configuration occurs, a central processing unit (hereunder referred to as a CPU) of the bar code reader 1 once receives an interrupt signal, which represents a control command corresponding to the communication interrupt, and determines whether or not the control command is directed to the bar code reader 1. If so, the bar code reader performs an interrupt processing according to the control command sent from the host system 1. Namely, when the host system 12 issues the interrupt signal to the bar code reader 1, the bar code reader should receive the interrupt signal once and perform the interrupt processing which includes the processing of determinating whether or not the control command represented by the interrupt signal is directed to the bar code reader 1. Thus the CPU of the bar code reader 1 cannot process image data during the communication interrupt. Consequently, the image data, which are originally continuous, are received by the bar code reader as discontinuous data, and as a whole the received image data become inaccurate.
Thus, when a communication interrupt occurs when the bar code reader 1 receives or reads image data, the conventional system does not treat the received image data as data and removes the received image data therefrom. Otherwise, the conventional system inhibits a communication interrupt thereto when image data are input thereto.
The conventional system of the former type has drawbacks in that the system removes image data even in case where the reading of the image data is almost completed, and thus should read the image data again later and that consequently, what is called a read completion rate is low.
Namely, in the conventional system of the former type, the slave device connected to the common signal line once receives an interrupt signal from the host system 12 and determines whether or not the received interrupt signal represents a control command directed to the slave device as above described. Practically, interrupts to the bar code reader 1 or the keyboard 14 take place very frequently. For example, in case where the number of the slave devices is two, interrupts occur almost 1000 times per second (namely, 500 (times/second).times.2 (slave devices)=1000 (times/second)). Thus, if the system once fails in reading image data, a very long time shall have passed by the time at which the next reading of the image data is completed.
Moreover, the conventional system of the latter type has drawbacks in that a communication interrupt indicated by a control command is not accepted by a slave device until an inputting of image data is completed, thus the slave device (for instance, the bar code reader 1) cannot quickly respond to a control command output from the host system 12 and, namely, the slave device cannot promptly respond as intended by an operator. The present invention is created to eliminate the above described drawbacks of the prior art.
It is, therefore, an object of the present invention to provide an information reading device which can increase a read completion rate by controlling a communication interrupt from a host system and also can quickly make a response as intended by an operator.